In most cases the solid substance on the TLC plate (called the "stationary phase") is quite polar. Often it is SiO2, called silica gel, which is actually very clean and finely ground sand. Polar substances tend to interact quite strongly with the polar stationary phase and so can be moved up the plate only by using a very polar solvent. Nonpolar substances interact weakly with the silica gel and move up the plate readily with nonpolar solvents. Suppose you have a mixture of the following solids: iodine (I2), benzophenone (H5C6-C(=O)-C6H5), benzoic acid (C6H5-C(=O)-OH), and oxalic acid (HOOC-COOH). You dissolve a small quantity of the solid mixture in ethanol and spot the solution on a TLC plate. You develop the plate using carbon tetrachloride, CCl4. Which of the 4 substances should have the largest Rf value? Which the smallest? Explain why.